In recent years, packaging materials have been predominantly prepared from composite flexible films made of different kinds of polymer materials in combination because of their strength, goods-protecting property, workability, advertising effects provided by printing or so, etc. The composite flexible films are generally constituted from a thermoplastic resin film, etc., which serves an outer layer for protecting goods, and another thermoplastic resin film, etc., which serves as a sealant layer. These layers are laminated together by a dry-lamination method in which the sealant layer is bonded to a laminated film layer through an adhesive applied to the laminated film layer, or by an extrusion lamination method in which a melt-extruded plastic film as the sealant layer is pressure-stuck with the laminated film layer which may be optionally coated with an anchor coat agent, thereby laminating the sealant layer over the laminated film layer in a form of film. In these methods, two-part liquid polyurethane-based adhesives that are generally composed of a main ingredient comprising an active hydrogen-containing group such as hydroxyl group, and an isocyanate group-containing curing agent, have been predominantly used as the adhesives in view of a high adhesion property thereof (for example, refer to Japanese Patent Application Laid-open Nos. Hei 5-51574 and Hei 9-316422, etc.).
However, these two-part liquid polyurethane-based adhesives generally exhibit a slow curing reaction rate. Therefore, in order to ensure a sufficient adhesion property of the two-part liquid polyurethane-based adhesives, the resultant laminated film must be aged for a long period of time, e.g., 1 to 5 days after the lamination, for promoting the curing reaction. Also, since the curing agent comprising isocyanate groups is used in the two-part liquid polyurethane-based adhesives, when residual unreacted isocyanate groups are present therein after curing, there may occur problems such as generation of bubbles in the resultant laminated film which is attributed to carbon dioxide formed by the reaction between the residual unreacted isocyanate groups in the adhesives and moisture in atmospheric air.
On the other hand, in order to overcome the above problems, Japanese Patent Application Laid-open No. 2000-154365 has proposed polyurethane-based adhesives, and WO 99/60068 has proposed epoxy-based adhesives for laminate.
However, the above polyurethane-based adhesives as well as the epoxy-based adhesives proposed in WO 99/60068 reveal a low gas-barrier property. Therefore, when these adhesives are employed for packaging materials requiring a high gas-barrier property, it is necessary to separately laminate additional various gas-barrier layers such as a PVDC coating layer, a polyvinyl alcohol (PVA) coating layer, an ethylene-vinyl alcohol copolymer (EVOH) film layer, a m-xylyleneadipamide film layer and an inorganic deposited film layer on which alumina (Al2O3), silica (Si) or the like is vapor-deposited, which results in high production costs of laminated films or disadvantageous laminating processes.